Low alloy ferritic steel for high temperature application



United States Patent LOW ALLOY FERRITIC STEEL FOR HIGH TEMPERATURE APPLICATION Michael Korchynsky, Niagara Falls, N. Y., assignor to- Union Carbide Corporation, a corporation of New 7 York No Drawing. Application July 18, 1956 Serial No. 598,530 9 Claims. (Cl. 75-123) Steel articles with properties permitting theireflicientuse at elevated temperatures under severe stress are required in steam generating plants, power turbines, chemical process equipment, as well as in jet and rocket engine parts. Such applications demand of the steel high creep strength, considerable resistance to oxidation and corrosion, practical ease of fabrication, and reasonably low costs.

Currently ferritic and austenitic steels are used in the above enumerated applications. Both steels present particular problems. In general, the use of ferritic steels is preferred at relativel moderate service temperatures, since ferritic steels manifest a rapid drop in creep resistance at temperatures of about 550 C. to 600 C.

Above this temperature range, austenitic steels normally are used, since they possess satisfactory creep strength at "ice An equally important object is to provide formed articles from such steels.

A further object is to provide a ferritic steel with properties in line with the above objects, which does not require special melting and hot working procedures.

A practical object of the invention is the provision of a low alloy ferritic steel capable of withstanding great stress at elevated temperatures for long periods of time, and which can retain its properties whether wrought, cast or investment cast, and can also be rolled to sheet thinness.

The invention by means of which these objects are achieved comprises the addition of small amounts of zirconium, molybdenum and boron to low carbon steel.

In the practice of the invention, additions of zirco- 'nium,'molybdenum or tungsten. and boron are made to steel in amounts aggregating from about 1.7 percent up to about 13 percent. A substantial excess of zirconium over that required to react with carbon, oxygen and nitrogen present in the steel is added. A convenient way of expressing this amount of zirconium is by a zirconiumto-carbon plus oxygen plus nitrogen ratio, which ratio must exceed about 8 to 1.

The heat treatment of the ferritic steels of the invention comprises solution treating at 1100 C. to 1300 C. to ensure the solution of zirconium in the matrix. The steels are subsequently cooled from this temperature at a rate suflicient to retain a super-saturated solution of a temperature up to 800 C. Undesirably, however,

higher costs accompany the use of austenitic steels.

Economical practice as well as technical factors, theredesirable to increase the temperature range of applicability of those steels up to about 650 C.

Prior art attempts to provide ferritic steels with improved properties suitable for high temperature application have included solution hardening of the steel matrix by the addition of such elements as molybdenum, tungsten, vanadium and manganese, and also by the precipitation of a dispersed hardening phase, generally by the formation of carbides. I

The above attempts to improve steel were based upon the fact that high temperature strength depends usually on the size, type and distribution of the precipitated hardening phase. It is also necessary that the dispersed phase in the steel matrix, whether it be carbide, nitride, boride or another, be sufiiciently geometrically stable to prevent rapid coalescence of particles, since otherwise the steel would be generally weakened. To that effect strong carbide formers such as tungsten, molybdenum,- vanadium, titanium or columbium conventionally are added to steel in which dispersed carbides are the strengthening phase. The better ferritic steels thus obtainable possess 1000-hour rupture strength of from 30,000 to 40,000 p. s. i. at 600 C.

Additions of boron to ferritic steel have also been suggested and tried, as it is well known that this element enhances creep resistanceat high temperatures. However, the high temperature strength which characterizes alloys thus modified does not exceed the levels of the steels mentioned above and, therefore, is not suflicient to meet the more severe present day requirements of various technologies.

With a view to overcoming the deficiencies of prior zirconium in the ferrite. The material then may be aged at 550 C. to 750 C., and air cooled. In this connection, the very low roomtemperature solubility of zirconium in steel (about 0.1), unlike that of the other elements which form inter-metallic compounds with iron, such as titanium, molybdenum, tungsten, tantalum and columbium, allows the use of minimum quantities of zirconium to produce precipitation hardening. Precipitation hardening may be caused by the formation and dispersion of Fe Zr.

Over and above iron impurities, including oxygen and nitrogen and elements normally found with steels and I steel alloy additions, the steels of the present invention contain the following:

Zirconium 0.2 to 3%.

Molybdenum and/or tungsten Up to 10% in the aggregate.

Boron 0.001 to 0.01%.

In the preferred embodiment of the invention steels contain the following:

Zirconium 0.4 to 1%.

Molybdenum and/or tungsten 0.5 to 1% in the aggregate. I

Boron 0.001 to 0.0025%.

- 0.7 percent manganese and 0.2 to 0.4 percent silicon.

art ferritic steels, it is the main object of this invention to provide low alloy ferritic steels which possess greater vperatures than heretofore produced.

Since zirconium carbides are not essential to the high temperature strength of these steels, carbon can be reduced to any practical minimum. Carbon is normally present in amounts ranging from 0.01 to 0.05 percent.

The melting of the alloys of the invention does not require special equipment or procedure, but conforms to normal metallurgical practice. In the preparation of the alloys of the invention, zirconium is added to the steel in the ladle in the form of sponge or as a master alloy after the initial deoxidation. Boron can be added as ferroboron with the zirconium, the presence of zirconium during this addition minimizing boron losses. denum may be added for example as ferromolybdenum.

An outstanding characteristic ofthese new steels is Molyb- 3 4- that they have high strength at high temperature. Eviently employed, the stress-rupture strengths of typical dencing this is the data ofTable'I following. 'ferritic and austenitic steels at 1100 F. (593 C.) are TABLE I Typical stress-rupturestrengths (p. s. i.) of zirconiumbearing steels 1 Composition 1,050 F. 505 0. '1,100F. 593 0. 1;200 F. (648 0.

Perzcent PefientPerEent 100-Hr. 1,000 Hr. 100 Ht. 1,000Hr. 100 Hr. 1,000 Hr.

C (1[eia1.tgl;ra)tment: Water, quenched from 1,250" O. (2,280 F.) and aged for 1 hour at000 Nbminal base compositlon'0.03% carbon, 0.70% manganese, 0.20% silicon, balance iron.

3 Cast sample. 4 Investment-east; sample.

Each figure represents the results of several tests, .and givenbelowin TablezII.

'TABLE II Iypical stress-rupture'strengths (-p. s. i.) of representative ferritic and austem'tic steels at 593 C. and 650 C.

593 0. 650 C. Type Composition .HeatATreatment 100 Hr. 1,000 Hr. 100 Hr. 1,000 Hr.

'Ferritlc' Steels Mn M0, 1.3 R1

Austem'tic Steels 0 Ni annealed 31,000

dn do 0.0 Ni x I .....d 0.0. N1, 1.5 Mo,-1-.25 V,-= O.Q.+Temp. 59,000 0.25 T1.

was obtained by the interor extrapolation of actual test AS can be seen, the alloy .of the invention compares very results obtained at various stress levels for the typical favorably With mapierlal 0f conslderably'lqlgher alloy c011- compositions of the ferritic steels shown. The results tent presently m use, even to the austenmc Steels- High-yield strength and yield ratio, both at room and tained on these steels, whether they are Wrought, cast, or eletvaled fi @gfi S2) dQLC fIQt 15 3-1 investment-cast. The ease of fabrication of the present ac erntlc S mo 1 e m a c r i t 11 0 s was roven hen in Ots wet rolled t heat f presentinvenhon. T e room temperature u tuna e ensl e a y P W g 5 and yield strengths are from 75,000 to 90,000 and 55,000

indicate that high stress-rupture properties can be obwith no difiicultyto 70,000 :p. s. i., respectively, and the amount of ductility the purposes Of compari on -0f the Stressupture is evidenced by 12 to 30 percent elongation. The table strength of the present alloys to those of materials presset forth below illustrates these properties.

TABLE III Typical short-time tensile properties of zirconium-bearing steels at room and elevated temperatures Composition Room Temperature 600 0. 700 0.

Percent Per- Percent Y. S., UTS, EL, R. A., Y. S., UTS, El. R. A., Y. S., UTS, EL, R. A.,

Zr cent B 1,000 1,000 Per- Percent 1,000 1,000 Per- Percent 1,000 1,000 Per- Percent Mo p. s. i. p. s. 1. cent p. s. i. p. s. 1. cent p. s. 1. p. s. 1. cent 0- 7 55. 8 73.2 30,0 62. 4 38.3 41.7 20.0 1.0 63.0 78.1 20.0 57.5 41.4 45.3 20.0 2. 0 69. 3 91. 0 12.0 18, 3 44. 51. 7 20. 0 30. 6 38 0 77.3 0.6 1 0 77.5 92. 2 f 22.0 60. G 52.0 55.3 16.0 38. 5 19.0 71, 7 0.7 84. 5 97. 2 20.0 61. 6 55.0 58. 5 13:0 48. 5 13. 0 53.8 0.7 1 0 p 71.0 86.5 20.0 '58. 5 55. 6 14.0 57.0 0. 7 1, 0 90. 8 102.0 14. 0 34. 7 65. 25 66. 3 12. 0 55. 5 11 0 '44. 0 0.7 1.0 90. 8 101. 4 10. 0 15. 3 62. 8 67. 2 10. 0

1 Heat-treatment: Water-quenched from 1250 0., aged 1 hour at 600 C.

2 Nominal base composition 0.03% carbon, 0.70% manganese, 0.20% silicon, balance n'on. 5 Aged 1 hour at 700 C.

Castsarnple.

# Investm0nt-cast sample.

Other alloying elements such as aluminum, silicon or chromium may be added to the present ferritic steels in quantities required to secuse certain desired degrees of oxidation or corrosion resistance without appreciably altering the increased high temperature strength.

What is claimed is:

1. A low alloy steel for high temperature application, containing carbon, oxygen and nitrogen in a proportion up to 0.25 percent; 0.2 percent to 3 percent zirconium, the ratio of said zirconium to carbon, oxygen and nitrogen where the same are present being at least 8 to 1; at least one member selected from the group consisting of molybdenum and tungsten in an amount in the aggregate up to percent; 0.001 percent to 0.01 percent boron; the remainder being iron and incidental impurities.

2. A low alloy steel for high temperature application containing carbon, oxygen and nitrogen in a proportion up to 0.05 percent; 0.40 to 1.00 percent zirconium, the ratio of said zirconium to carbon, oxygen and nitrogen where the same are present being at least 8 to 1; at least one member selected from the group consisting of molybdenum and tungsten in an amount in the aggregate between .05 and 1.0 percent; 0.001 to 0.0025 percent boron; the remainder being iron and incidental impurities.

3. A low alloy steel for high temperature application, containing carbon, oxygen and nitrogen in a proportion up to 0.15 percent; 1 to 3 percent zirconium, the ratio of said zirconium to carbon, oxygen and nitrogen where carbon, oxygen and nitrogen are present being at least 8 to 1; 1 to 10 percent in the aggregate of molybdenum and tungsten; 0.0025 to 0.01 percent boron; the remainder being iron and incidental impurities.

4. A low alloy steel for high temperature application, containing carbon, oxygen and nitrogen in a proportion up to 0.25 percent; about 2 percent zirconium, the ratio of said zirconium to carbon, oxygen and nitro gen where the same are present being at least 8 to 10; 0.5 to 10 percent in the aggregate of molybdenum and tungsten; 0.001 to 0.01 percent boron; the remainder being iron and incidental impurities; said zirconium being present in the form of the inter-metallic compound Fe Zr dispersed throughout the alloy matrix.

5. A low alloy steel for high temperature application containing up to 0.03 percent carbon, 0.7 percent zirconium, the ratio of said zirconium to carbon where carbon is present being at least 8 to 1; at least one member selected from the group consisting of molybdenum and tungsten in an amount in the aggregate up to 1.0 percent; 0.004 percent boron; 0.20 percent silicon; 0.70 percent manganese; the remainder being iron and incidental impurities; said zirconium being present in the form of the inter-metallic compound Fe Zr dispersed throughout the alloy matrix.

6. A low alloy steel for high temperature application, containing up to 0.03 percent carbon, 0.5 percent zirconium, the ratio of said zirconium to carbon where carbon is present being at least 8 to 1; 1.0 percent in the aggregate of molybdenum and tungsten; 0.002 percent boron; 0.20 percent silicon; 0.70 percent manganese; the remainder being iron and incidental impurities; said zirconium being present in the form of the inter-metallic compound Fe Zr dispersed throughout the alloy matrix.

7. A low alloy steel for high temperature application, containing up to 0.03 percent carbon, 0.7 percent zirconium, the ratio of said zirconium to carbon where carbon is present being at least 8 to 1; 1.0 percent in the aggregate of molybdenum and tungsten; 0.002 percent boron; 0.20 percent silicon; 0.70 percent manganese; the remainder being iron and incidental impurities; said zirconium being present in the form of the inter-metallic compound Fe Zr dispersed throughout the alloy matrix.

8. A low alloy steel for high temperature application containing up to 0.03 percent carbon, 0.6 percent zirconium, the ratio of said zirconium to carbon where carbon is present being at least 8 to 1; 1.0 percent in the aggregate of molybdenum and tungsten; 0.002 percent boron; 0.20 percent silicon; 0.70 percent manganese; the remainder being iron and incidental impurities; said zirconium being present in the form of the inter-metallic compound Fe Zr dispersed throughout the alloy matrix.

9. In a method of producing a low alloy steel for high temperature applications, which alloy contains zirconium dispersed throughout the alloy matrix, the steps consisting of adding small amounts of zirconium, molybdenum and boron to low carbon steel; solution treating at a temperature range of 1100 C. to 1300 C., cooling from said temperature range at a rate sufiicient to retain a super-saturated solution in the matrix, aging at a temperature range of 550 C. to 750 C. and air cooling the alloy.

References Cited in the file of this patent UNITED STATES PATENTS 2,542,220 Urban Feb. 20, 1951 FOREIGN PATENTS 373,017 Great Britain May 19, 1932 OTHER REFERENCES The Journal of the Iron and Steel Institute (British), No. 1, 1954, vol. 176, page 467.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 2,871,117 January 27, 1959 Michael Korchynsky It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 5, line 38, for 'at least 8 to 10" read at least 8 to 1. a

Signed and sealed this 2nd day of June 1959,

SEAL) Attest:

KARL H, AXLINE ROBERT C. WATSON Attesting Officer Commissioner of Patents 

2. A LOW ALLOY STEEL FOR HIGH TEMPERATURE APPLICATION CONTAINING CARBON, OXYGEN AND NITROGEN IN A PROPORTION UP TO 0.05 PERCENT; 0.40 TO 1.00 PERCENT ZIRCONIUM, THE RATIO OF SAID ZIRCONIUM TO CARBON, OXYGEN AND NITROGEN WHERE THE SAME ARE PRESENT BEING AT LEAST 8 TO 1; AT LEAST ONE MEMBER SELECTED FROM THE GROUP CONSISTING OF MOLYBDENUM AND TUNGSTEN IN AN AMOUNT IN THE AGGREGATE BETWEEN .05 AND 1.0 PERCENT; 0.001 TO 0.0025 PERCENT BORON; THE REMAINDER BEING IRON AND INCIDENTAL IMPURITIES. 